DESCRIPTION (Adapted from applicant s abstract): Background. Ligation of CD28 provides potent costimulation for responses initiated through the T cell receptor (TCR) increasing cytokine production as well as preventing desensitization, anergy, and activation-induced cell death through apoptosis. Thus, one of the potential mechanisms resulting in failure to eliminate tumors in cancer patients is a lack of appropriate activation of CD28. Indeed, in animal models introduction of the B7 ligand for CD28 into tumor cells can induce an effective long-lasting anti-tumor response. This paradigm is now being tested as a tumor vaccine in human clinical trials at MD Anderson Cancer Center and elsewhere. An understanding of the biochemical signals initiated by which CD28 augments T cell responses may allow the development of more effective therapeutic approaches to cancer treatment or alternatively to the identification of small molecules able to mimic the effect of CD28 crosslinking in the development of effective and practical tumor vaccines. Preliminary Data. The applicant has shown that the EMT/ITK/TSK (EMT) intracellular tyrosine kinase, cloned in the laboratory, is activated following crosslinking of CD28. Preliminary data indicate that EMT constitutively and inducibly associates with CD28 and phosphatidylinositol 3' kinase (PI3K). The constitutive association of EMT with CD28 appears to be mediated by an interaction between the SH3 domain of EMT and proline rich domains in CD28. The applicant has shown that activation of CD28 prevents apoptosis induced by crosslinking of the TCR both in in vivo and in vitro, at least in part, through the induced expression of BCLx. Preliminary data indicate that crosslinking of CD28 can inhibit anti-FAS mediated apoptosis in Jurkat T cells. Rationale. That by investigating CD28 signal transduction through EMT, one can obtain an improved understanding of the biochemical mechanisms by which CD28 regulates the immune response eventually leading to development of new effective methods to modulate the immune response in cancer patients. Approach. Site-directed mutagenesis of CD28 and EMT will be used to characterize potential interaction sites and to determine their roles in EMT activation and kinase activity and in CD28 signal transduction. Specific Aims. 1. To determine the mechanisms by which EMT constitutively and inducibly associates with CD28 and PI3K. 2. To determine the mechanisms regulating the activation of EMT. 3. To determine the mechanisms by which CD28 regulates physiological responses.